1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of halogen substituted aromatic hydrocarbons. More specifically, the invention relates to a process for separating para and ortho isomers of halotoluenes employing as the adsorbent X or Y zeolites containing alkali or alkaline earth metal cations.
2. Background Information
A wide variety of halogenating agents, catalysts and reaction conditions are used in processes for halogenating toluene. Thus far the isomers of chlorotoluene have been of the most interest, accordingly, most information related to the halogenated toluenes are in the context of the chlorotoluene isomers. Therefore in looking at the preparation and specific properties of halotoluene isomers, specific reference will be made to chlorotoluene. However, the procedures and information related to the other halogen substituents is similar in most respects. In regard to chlorotoluene isomers, most recent work has attempted to develop means for increasing the yield of the para isomer which has greater commercial significance. Mixtures of monochlorotoluenes are obtained by chlorination with certain Lewis catalysts including the chlorides of aluminum, tin, titanium and zirconium. Through these procedures the mixture of monochlorotoluenes will contain more than 70% of the ortho isomer. A para isomer content in the range of 45-55% is obtainable through the use of metal sulfide cocatalyst systems containing metal salts and sulfur, inorganic sulfides or divalent organic sulfur compounds. Other methods of obtaining monochlorotoluene isomers include the noncatalytic nuclear chlorination of toluene in various solvents, the reaction of toluene with chlorinating agent comprising certain Lewis acid halides and the use of hydrogen chloride as a chlorinating agent in both liquid and vapor phase systems. When preparing monochlorotoluene isomers by direct chlorination it has been found that less than 1% of the isomers produced will comprise the meta isomer. Consequently feedstocks for this invention will often contain a small proportion of the meta isomer which is not removed prior to the separation of the para and ortho isomers.
Separation of ortho and parahalotoluenes is difficult due to the close boiling point range of these isomers. In order to accomplish a separation of these isomers by fractionation it is necessary to use a high efficiency isomer separation column. This invention simplifies the separation procedure by providing a more effective adsorptive separation method.
Crystalline alumina silicates are commonly used in the separation art to perform adsorptive separations. Example of such separations in the field of hydrocarbons are disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. In these patents normal paraffins are separated from branched chain paraffins using a type A zeolite. U.S. Pat. Nos. 3,265,750 and 3,510,423 are more specifically directed to the use of faujasites to separate olefinic hydrocarbons from paraffinic hydrocarbons. Adsorbents such as these rely on physical size differences in the molecules to perform the separation wherein the smaller or normal hydrocarbons are allowed to pass into cavities within the zeolite adsorbent which the larger or branched chain molecules cannot enter. The use of X or Y zeolites to separate individual hydrocarbon isomers is also well known. For example, U.S. Pat. Nos. 3,626,020, 3,663,638, 3,665,046, 3,668,266, 3,686,343, 3,700,744, 3,734,974, and 3,997,620 demonstrate the use of zeolitic adsorbents in the separation of the para isomer of alkyl substituted monocyclic aromatics from other isomers particularly paraxylene from other xylene isomers.
Turning now specifically to the separation of substituted aromatic isomers there are several relevant patent publications. The separation of substituted benzene isomers using a faujasite containing silver or copper cations is taught in German Patent DE No. 3327146. More specifically related to the present invention are the public disclosures related to Japanese Patent Application Nos. 11,884/82 and 50,440/83. Both of these patent applications deal with the separation of metachlorotoluene from a mixture of chlorotoluene isomers. In the first application metachlorotoluene is adsorbed on a Y zeolite containing silver and potassium cations. The second application teaches the adsorption of metachlorotoluene on a Y zeolite adsorbent containing sodium and copper cations as essential components.
With suitably active catalysts it is possible to obtain halogenated toluene isomers with only trace amounts of side chain halogenated products. Nevertheless there would usually be a small percentage of reactive halides which are left over from the preparation of the isomers. Traces of halides, in particular chloride in the product, can be highly reactive with any transition metal ions that are exchanged into the zeolite adsorbent. Therefore in light of this likely contamination it is highly desirable to avoid the use of ions such as copper or silver which will readily react with the traces of chloride or other halides that may be present in the feedstock.
The separation of halogenated aromatic isomers using zeolites containing alkali or alkaline earth metal cations is disclose in U.S. Pat. No. 2,958,708. This reference includes examples demonstrating the relatively low selectivity of a calcium exchanged zeolite for orthochlorotoluene over parachlorotoluene using a chlorobenzene displacement fluid.
In contradistinction to the prior art, the present invention resides in a particular combination of adsorbents and desorbents which substantially increases the selectivity of the adsorbent for one halogenated toluene isomer over another.